In a thermal power plant or a nuclear power plant connected to an electric power system, when the plant is in a normal operation, a mechanical input from a turbine and an electrical output of a generator are kept in balance, so that the turbine rotation speed is maintained constant.
However, if an accident occurs in the electric power system, this balance is lost due to a rapid decrease in electrical output of the generator, and the turbine rotation speed increases. If this speed reaches a certain limit, a stable operation of the plant may become no longer possible.
Early Valve Actuation is known as a method for preventing such circumstances. This technique suppresses increase in the turbine rotation speed, in the event of electric power system accident, by fully rapid closure of control valves or intercept valves, which flow steam into the turbine, and thereby temporarily blocking the steam flow into the turbine.
There has been another method disclosed, which responds to an electric power system accident by reducing the valve opening position of control valves and intercept valves so as to adjust the flow rate of steam into a turbine (e.g., see Patent Document 1).